The present invention relates to an apparatus for proportioning or for proportioning and mixing plural different fluid compositions, particularly two different fluid compositions which cure when mixed together and which have different degrees of compressibility.
More particularly, the present invention is directed to such an apparatus employable for the storage, shipping, proportioning, mixing and dispensing of at least two such different fluid compositions.
Unlike hydraulic liquids, most single and multicomponent adhesives, sealants and mastics contain a certain amount of entrained air which becomes mixed into the composition or compositions during formation or compounding thereof. The level of such entrained air may be tolerated or controlled for technical and commercial reasons. Additionally, some forms of fillers employed in such compositions themselves may be compressible. Examples of such compressible fillers are microglass spheres or balloons. Furthermore, air bubbles may be trapped within a cartridge containing the component or components during loading thereof into the cartridge, and such trapped air bubbles will compress and expand during dispensing cycles. The cartridge body may expand and contract due to supported wall clearances or unsupported wall movement.
The result of the above is that, in cooperation with back pressure from the nozzle, some degree of compression of the component or components takes place during the application of pressure to the component or components to dispense the same from the cartridge. Thus, after the power source of dispensing pressure is removed, the component or components are often seen to "run-on" from the nozzle for a certain period of time while decompression takes place. This "run-on" may be used up as part of the material being dispensed for a particular operation or may be allowed to run to waste.
Generally, such run-on does not seriously affect the performance of a cartridge for dispensing a single component material, and the characteristic run-on is accepted as being overridden by the benefits of such a device.
On the other hand, with multicomponent or composition materials, the problem of run-on is multiplied and much more complex, with damaging results to the chemistry of the multicomposition material.
When proportioning only or when proportioning and mixing plural liquid compositions such as reactive adhesive systems from a cartridge, for example a coaxial cartridge arrangement, the chances are high that one fluid composition or material will run-on to a greater or lesser extent than the other compositions or materials. This is due to the factors described above, and additionally due to the fact that the proportioning ratio between the plural compositions adds another factor to the likelihood of unequal compression/decompression. Unlike run-on of a single component material, the results of unequal run-on from the materials of a multicomponent material are that the proportional ratio of the plural compositions at some point in time becomes incorrect. This causes "off ratio" patches to be dispensed from the cartridge and mixing device, for example an in-line motionless mixer. The results of numerous tests involving multiple cartridge dispensing of various two component reactive systems have determined that only a few such systems remain within tolerance limits of the desired predetermined proportional ratio of mixture of the two components during the starting and stopping periods of flow, and that most such systems result in "off ratio" patches, either visibly or by their performance. Since some specific skill or test procedure would be necessary to control the quality of the mixed composition resulting from the multiple compositions at the starting and stopping points of flow thereof, the benefits of the overall device are reduced, sometimes to the point of being completely unacceptable in practice. Thus, a user dispensing a long bead of adhesive may scrap the first dispensed volume until a visual color change indicative of a proper predetermined proportional ratio has occurred. However, a lower volume user wishing to start and stop for small shots of the mixed composition easily might have his production efficiency substantially reduced by off ratio mixtures. Furthermore, many reactive composition systems are such that there does not occur a visible color change between off ratio and on ratio mixtures. This further adds to the uncertainty of attempting to determine when the mixture is on ratio.
This problem may be reduced to some extent by increasing the mixer and nozzle size, thereby reducing the outlet back pressure. However, large mixers and nozzles generally are not desirable in this type of system.
A more specific explanation of the problem is as follows. Thus, in a two component or composition cartridge system, upon the application of pressure to achieve dispensing, the less compressible material starts flowing through the mixer before the other material while such other material still is compressing. As the compression of the other, more compressible material is increased, the flow of such other material increases to a point where the two materials flow through the mixer in the desired predetermined proportional ratio. Therefore, after an initial off ratio mixture, high proportion of the less compressible material, the mixed composition becomes correct. As soon as the dispensing pressure is removed the less compressible, more hydraulic material ceases to flow more immediately, while the more compressible, less hydraulic material continues to flow, i.e. to run-on. At this point, the more compressible, less hydraulic composition predominates in the mixed composition, and the extent of this predomination is the volume of the more compressible material which should have flowed forward with the less compressible material during the initial starting operation to achieve the desired predetermined proportional ratio. This stopping or final run off now is off ratio. Assuming that the unit is at rest for a period of time to allow complete decompression to take place, it is likely that the mixer will contain an incorrectly high proportion of the more compressible, less hydraulic composition. Therefore, when pressure again is applied to commence dispensing, there first will occur a discharge of a mixture having a high proportion of the more compressible component, and then there will occur discharge of a mixture having a high degree of the less compressible component. The above sequence of events will be repeated during each cycle of starting, stopping and then again starting dispensing, with the exception that upon continued use, the reduced volumes left within the chambers of the cartridge will cause a reduction in the extent of compression/decompression. Economic cartridge sizes are such that the volumes always will be high enough to reflect the above discussed occurrences of off ratio mixtures.
It of course will be understood that it also is desirable in an apparatus for dispensing at least two different fluid compositions that the apparatus be operable in such a manner that flow can be stopped and started immediately with no run-on, whereby small dots, shots of beads can be dispensed accurately.